Perturbative unitarity in quasi-single field inflation

TL;DR

This paper investigates the constraints of perturbative unitarity on quasi-single field inflation, linking non-Gaussianity levels to the scale of new physics and analyzing the viability of models with heavy isocurvature modes.

Contribution

It establishes a relation between non-Gaussianity amplitude and the UV cutoff scale, and clarifies the parameter space for UV-completable inflation models with heavy scalars.

Findings

High non-Gaussianity requires interactions that violate unitarity at high energies.

Planck-scale suppressed operators can produce excessive non-Gaussianities at high Hubble scales.

The parameter space for UV-complete $P(X,\phi)$ models with a heavy scalar is delineated.

Abstract

We study implications of perturbative unitarity for quasi-single field inflation with the inflaton and one massive scalar. Analyzing high energy scattering, we show that non-Gaussianities with $|f_{\rm NL}|\gtrsim1$ cannot be realized without turning on interactions which violate unitarity at a high energy scale. Then, we provide a relation between $f_{\rm NL}$ and the scale of new physics that is required for UV completion. In particular we find that for the Hubble scale $H\gtrsim 6\times 10^{9}$ GeV, Planck suppressed operators can easily generate too large non-Gaussanities and so it is hard to realize successful quasi-single field inflation without introducing a mechanism to suppress quantum gravity corrections. Also we generalize the analysis to the regime where the isocurvature mode is heavy and the inflationary dynamics is captured by the inflaton effective theory. Requiring perturbative unitarity of the two-scalar UV models with the inflaton and one heavy scalar, we clarify the parameter space of the $P(X,\phi)$ model which is UV completable by a single heavy scalar.